The progressive improvement in the quality of magnetic recording playback units, as well as the development of more complex signal processing capability in such units, have all served to place increased demands upon the quality of the recorded information placed upon magnetic tape. Since the quality of such information is intimately related to the magnetic properties of the tape itself, correspondingly increased demands have been placed upon the properties of magnetic tapes in general. In particular, increased demands are placed on such materials so as to provide for improved frequency response, signal to noise ratio, as well as channel separation for stereophonic or quadraphonic sound reproduction. Also, the increased use of tape cassettes for minicomputers has placed an additional public demand for extra-high quality tape.
It is a general objective in the fabrication of audio magnetic tape recording to produce a tape which is easily erasable, which means that it provides a coercive force low enough to be easily erased but high enough to avoid print-through between adjacent overlying wraps of tape wrapped around a dispensing and take-up spool.
It had been observed in the fabrication of ferrite permanent magnets used as loud speaker coil magnets, where permanence rather than erasability of the magnetized material is desired, that the admixture of a few percent of samarium pentoxide as a surface coating agent for the iron oxide particles forming the magnet resulted in improvement in the resulting magnet. In the process of fabricating the magnet, a powder mixture of the samarium pentoxide dispersed through the powdered iron oxide as a surface coating thereon was placed into a press, wherein the powder mixture was held in an appropriate form, and a magnetic field was applied to the confined mixture to provide a consistent grain orientation of the iron oxide crystals. This orientation was greatly improved as the direct result of the samarium coating, permitting the micron size ferric oxide platelets to better align each to the next. Pressure was applied to the powder matrix, and the matrix was sintered at high temperatures to form a self-supporting solid structure with a resulting improved permanent magnet.
The exact reason why samarium pentoxide in such small admixtures gives such improvement in the magnetic properties of these permanent magnets remains obscure. However, it was not heretofore considered useful to incorporate, as in the case of the present invention, a rare earth-containing material like samarium as a coating on the iron oxide particles forming an erasable magnetic recording medium for any purpose, let alone for the purpose of improving frequency response, signal to noise ratio, or channel separation of a magnetic audio recording tape.
The present invention involves the recognition that a rare earth-containing material, preferably samarium oxides in the preferred form of the invention, and other rare earth materials, like cerium, are useful as coatings of the magnetic recording material particles involved in erasable magnetic audio recording tapes and the like, significantly to improve frequency response, signal to noise ratio and/or channel separation therein, without significantly adversely affecting the erasability of such tapes. Such results are achieved when the rare earth-containing material is no more than about 2% and preferably less than about 1% of the weight of the coated magnetic particles involved. The rare earth-containing material is most advantageously samarium pentoxide.
The coating material is initially ball milled to a very fine powder. Next, it is mixed with the particles of magnetic recording material involved, preferably iron oxide, in very small amounts, and is then ball milled therewith to form a coating thereover. This mixture may be added to a standard uncured magnetic tape binder before being coated on a backing material, or applied with the binder upon one side of the backing material. The applied coating is subjected to a magnetic field to orient the particles of the recording material, and then the resultant coated tape is dried and cured in a conventional way.
The use of samarium compounds can be found in magnetic recording mediums for permanent magnetic documents like credit cards where a permanent not easily erasable rather than an easily erasable magnetic record is desired. Thus, there is disclosed in an article entitled "Magnetic Recording Properties of SmCo.sub.5 " by Fayling and Benson, IEEE Transactions on Magnetics, Volume Mag. 14, No. 5, December, 1978, the use of SmCo.sub.5 as the recording medium (i.e. the samarium was not used as a coating for iron oxide (Fe.sub.2 O.sub.3) magnetic particles). Also, the extremely high coercive force desired for this application makes such a disclosure completely inapplicable to erasable magnetic audio recording tapes.
Samarium has also been used as a composition element in magnetic material compositions, as distinguished from a coating on particles of iron oxide or the like. U.S. Pat. Nos. 4,145,301, 4,165,410, 4,047,982, 4,034,134, and 3,949,386 are examples where samarium is such an element.
U.S. Pat. No. 4,034,133 discloses the use of samarium trifluoride as a lubricant in a magnetic recording tape comprising magnetic particles dispersed in a polymeric binder. The samarium trifluoride is dispersed either throughout the binder-magnetic material composition or as a separate coating on the top thereof. The samarium trifluoride utilized is disclosed as being from 11/2% to 50% by weight of the entire coating (i.e. including the binder). There is no recognition of the use of the samarium trifluoride for a purpose other than as a lubricant (i.e. to reduce friction) with a magnetic recording or reproducing head).